This invention addresses the need for providing a distortion-free image of a rounded surface, such as a fingerprint. The apparatus belongs to the category of livescan fingerprint scanners, as it directly acquires the image of a fingerprint in contact with a clear surface, bypassing the requirement for a staining fluid as found in conventional ink and roll procedures.
There is already a body of prior art concerning the inkless imaging of fingerprints. Most such systems utilize the critical angle phenomenon and/or the dual phenomenon of total internal reflection. A generic device which demonstrates readily both concepts is a right angle glass prism. A finger is placed on the hypotenuse or posterior surface of the prism, and optical pickup occurs along an axis in front of and perpendicular to one of the other two prism surfaces, denoted here as the front surface. This axis, when extended to the back surface, forms an angle with its normal which is greater than the critical angle of light entering the glass from a medium with unity index of refraction, such as air. When a finger is placed in contact with the posterior prism surface, air is trapped between the papillary valleys and glass, and all light arising from the papillary valleys which penetrates the glass is refracted away from the aforementioned perpendicular axis. Light scattered from regions of the finger (papillary ridges) in direct contact with the glass does not undergo such a degree of refraction, due to the absence of the air-glass interface. Light scattered from the papillary ridges can therefore find a path to the photodetector device. If no light is reflected internally off regions of the posterior surface in contact with air, then the papillary ridges will appear illuminated relative to parts of the contact surface underlying the papillary valleys, and the dominant operating principle is the critical angle effect. If illumination is provided such that light is (totally) internally reflected off regions of the posterior surface in contact with air, then the parts of the contact surface directly underlying the papillary valleys will appear more illuminated with respect to the papillary ridges, which tend to frustrate the total internal reflection by absorbing some light and by scattering the remainder omnidirectionally.
Devices have been proposed based upon this prism approach. A disadvantage of the flat posterior surface is that only a limited portion of the otherwise curved finger surface can make contact and hence be imaged. One method for circumventing this problem is to form a well, generally cylindrical, in the posterior surface of the prism. The well accommodates the curvature of the object to be imaged, yet has the drawback of providing poor visualization of those portions of the well whose normal vector forms a large angle with the normal vector of the front prism surface. In addition, distortion of the image occurs when the light rays emanating from the curved object surface are focused onto a flat detector surface.
Solutions to the problem of complete and distortion-free imaging of the cylindrical surface, while still using the critical angle effect or frustrated total internal reflection, have been proposed which depart from the use of a prism per se. At least one such method employs a cylindrical transparent platen into which is inserted the finger, and under and around which a light source and sensor device, together with a focusing mechanism, are revolved, resulting in the acquisition of modulated light corresponding to the pattern of papillary ridges and grooves. The light source is directed at such an angle to the cylindrical well that light from it is totally internally reflected toward the photodetector, excepting where the presence of a papillary ridge in contact with the well frustrates this effect.
A disadvantage of an assembly such as is mentioned in the foregoing paragraph is the necessity to revolve the light source and focusing lens in synchrony with the photodetector. The invention disclosed in this patent uses a stationary light source and focusing lens. Embodiments are discussed which allow the source to be positioned such as to yield an imaging system based on either the critical angle effect or on frustrated total internal reflection.